1. Field of the Invention
The present invention relates generally to a discharge tube, and more particularly to a discharge tube which discharges between a discharge plane of an upper discharge electrode and a discharge plane of a lower discharge electrode, at the center of an airtight cylinder.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. 10-335042 shows an example of a discharge tube used in, for example, a ballast circuit for igniting an HID (High Intensity Discharge) lamp of a vehicle, or an igniter circuit for igniting a rear side lamp of a liquid crystal projector. This discharge tube (discharge tube 1A), shown in FIGS. 1 to 3, includes, for example, an airtight cylinder 10 formed into a cylindrical shape from an insulating material, an upper discharge electrode 22 joined to an upper opening part of the airtight cylinder 10 shown in FIG. 1, and a lower discharge electrode 24 joined to a lower opening part of the airtight cylinder 10 shown in FIG. 1.
Four main discharge trigger wires 80, being transversely formed at a middle section of the inner wall of the airtight cylinder 10, are aligned with a prescribed interval in an upright manner in a direction parallel to the axis of the airtight cylinder 10. Four sub-discharge trigger wires 90, being formed at upper and lower sections of the inner wall of the airtight cylinder 10, are aligned between the main discharge trigger wires 80 in an upright manner in a direction parallel to the axis of the airtight cylinder 10. The upper and lower ends of the sub-discharge trigger wires 90 are connected to a metallized plane 40 formed on an upper end surface and a lower end surface of the airtight cylinder 10.
In the discharge tube 1A, sputter, being created during discharge from a discharge plane of a distal end of the upper discharge electrode 22 (hereinafter referred to as “upper discharge plane 23”) and a discharge plane of the lower end of the lower discharge electrode 24 (hereinafter referred to as “lower discharge plane 25”), adheres to the middle section of the inner wall of the airtight cylinder, to thereby prevent the insulating properties of the main discharge trigger wire 80 and the sub-discharge trigger wire 90 from deteriorating. Therefore, an electrical discharge can be repeatedly and steadily induced at a prescribed potential for a long period between the upper discharge plane 23 and the lower discharge plane 25.
FIG. 7 shows the data of the electrical discharge property of the discharge tube 1A in a case where a single unit of the discharge tube 1A is activated under an ideal environment that is free from being affected by, for example, an external magnetic field. FIG. 7 shows that a stable arc discharge is repeatedly induced in a regular manner. It is to be noted that the horizontal axis indicates time and the vertical axis indicates discharge voltage (1 square space represents 1000 V) in FIG. 7 and also in the other diagrams given below that show data of an electrical discharge property.
Along with the accelerating trend of mobilization of liquid crystal projectors in recent years, electronic devices and circuits that are mounted to the liquid crystal projectors tend to be size-reduced and disposed in high density. Further, respective electronic devices and electronic components are also packaged in high density.
The discharge tube 1A may be, therefore, disposed in the proximity of a booster coil 2 (hereinafter referred to as “coil”) (see FIG. 4) that serves to boost an AC 100V commercial power supply to 3000V or more, or in the proximity of a power transformer 4 (hereinafter referred to as “transformer”) (see FIG. 5). Furthermore, the discharge tube 1A may be situated between the coil 2 and the transformer 4 depending on the layout of the components.
By disposing the discharge tube 1A proximal to the coil 2 or the transformer 4 in the manner described above, the coiling direction of the coil 2 or the transformer 4 will be substantially perpendicular to the main discharge trigger wire 80 or the sub-discharge trigger wire 90 of the discharge tube 1A. This causes the magnetic field or the electromagnetic waves generated by the coil 2 and the transformer 4 to affect the discharge tube 1A, and create an electric current in the main discharge trigger wire 80 and the sub-discharge trigger wire 90 by electromagnetic induction. As a result, the discharge tube 1A is liable to malfunction.
An experiment for obtaining electrical discharge property data was conducted in a case where: {circle around (1)} the discharge tube 1A is situated in the proximity of the coil 2 (a coil boosting the AC 100V commercial power supply to 4000V or more) as shown in FIG. 4, {circle around (2)} the discharge tube 1A is situated in the proximity of the transformer 4 as shown in FIG. 5, and {circle around (3)} the discharge tube 1A is situated between the coil 2 and the transformer 4 as shown in FIG. 6, respectively.
It is to be noted that the term “proximity” in the experimental cases of {circle around (1)} and {circle around (2)} refers to a position that is no more than 2 mm apart from the coil 2 and the transformer 4.
FIG. 8 shows the electrical discharge property data in the case of {circle around (1)} where the discharge tube 1A is situated in the proximity of the coil 2, FIG. 9 shows the electrical discharge property data in the case of {circle around (2)} where the discharge tube 1A is situated in the proximity of the transformer 4, and FIG. 10 shows the electrical discharge property data in the case of {circle around (3)} where the discharge tube 1A is situated between the coil 2 and the transformer 4.
In comparing the electrical discharge property data of the discharge tube 1A in FIG. 7 with the electrical discharge property data of the discharge tube 1A in FIGS. 8 to 10, the data in FIGS. 8 to 10 show an unstable electric potential of discharge that largely fluctuates.
Such instability and fluctuation are caused by the discharge tube 1A being affected by the electric field and the electromagnetic wave generated from the coil 2 and the transformer 4.
This large fluctuation may become the cause for miss-fire and may prevent the discharge tube 1A from suitably driving, for example, a liquid crystal projector.